Bayern Munich coach Pep Guardiola believes Lionel Messi is unstoppable on current form as he prepares to face his old club Barcelona for the first time in their Champions League semi-final, first-leg on Wednesday.

Messi and Guardiola won 14 trophies together during the latter’s glorious four-year spell in charge between 2008 and 2012.

But the Spaniard insists even he doesn’t have the formula to stop the four-time World Player of the Year, who has scored 51 goals in 50 games this season.

“There is no defence that can stop Messi. It is impossible,” Guardiola said on his return to the Camp Nou wearing the colours of the opposition for the first time.

“How he is playing at the moment, you cannot stop him.

“Defending with nine in your area or 11 pressing high. You can’t always be at your best physically during your career, there are ups and downs, but the way he has played the last four or five months there is no way to stop him. He is too good.

“Obviously you have to get close and try to ensure he receives the ball less, but there is not a system or coach to stop talent of his magnitude.”

Messi said earlier in the day that Guardiola would rightly receive a hero’s welcome before the game for the 15 years service he gave the club firstly as a player and then as a coach.

However, Guardiola insisted that his return home is strictly business despite having to withstand a wave of unusual emotions ahead of a game of this magnitude.

“It is great to be here, there are so many memories and it is a big moment for me.

“But I am here as the coach of Bayern Munich and I have come for a game of football, that is my job, my profession and I have to prepare for the game tomorrow as best as I possibly can.

“When you coach big teams like Bayern Munich, sooner or later this is going to happen. For me it is the first time, but I am not here for a homage, I am here to do my job and get Bayern Munich to the final.”

Guardiola also has more than Messi to worry about as the Argentine forms just a third of a fearsome front line also encompassing Luis Suarez and Neymar that have scored 108 goals already this season.

Whilst Barca come into the game at full-strength, having won 26 of their last 28 games to move to the bring of a treble, Bayern have been rocked by a series of key injuries.

Arjen Robben, Franck Ribery, David Alaba and Holger Badstuber all miss out, whilst Guardiola said he will speak with Robert Lewandowski to see whether the Pole is comfortable enough to play with a protective face mask after breaking his nose and cheekbone as Bayern were dumped out of the German Cup by Borussia Dortmund on penalties last week.

However, Guardiola strongly refuted the suggestion that he would ever use the German champions’ lengthy injury list as an excuse.

“All coaches want to go into the games in the best condition possible, but during the season things happen.

“We will prepare for the game with the players that we have as we have no other option.

“Never in my time here or as Bayern coach will you hear me say that we didn’t win because of the players that weren’t there.”